Prior activity in the development of low NO.sub.x burners has focused on cooling the burner flame by steam or water vapor to reduce NO.sub.x production. Flue gas has also been helpful in cooling of the burner operation. Means to increase the amount of flue gas around the burner should be encouraged. In Reed, U.S. Pat. No. 4,089,639 a burner is provided to reduce NO.sub.x which involves premixing of preheated water with fuel prior to the burning operation. Means are provided for spraying preheated water into the fuel so that water vapor will be taken up by the heated fuel in sufficient quantities to provide for a cooler burning flame and, thus, lower NO.sub.x emissions. This system is sometimes referred to as "convergent" burning and is exemplified in FIG. 1 of this application. At least four different burner embodiments are shown in the Reed patent in FIGS. (1A and B), (2A, B and C), (FIG. 3), and (FIG. 4).
A combustion process for a coal fired boiler is described in Welty, U.S. Pat. No. 3,463,599, where coal is burned so that no oxygen or sulfur trioxide is present in the resultant flue gas. A sufficient amount of additional hydrocarbons is charged to the flue gas to burn excess oxygen. By adjusting the stoichiometric quantity of hydrocarbons to oxygen, NO.sub.x is reduced. A method for burning coal dust in the presence of water vapor is described in Ottensoos et al, U.S. Pat. No. 3,598,314 which comprises heating the coal dust and water prior to passage to a nozzle at pressures and temperature conditions of just above water-vaporizing conditions so that water is pressure-relieved and vaporizes after passage through the nozzle. A heat exchange system is disclosed in Frondorf, U.S. Pat. No. 3,938,934 where combustion air fed to a fuel burner is warmed by indirect contact with warm water.